1. Field of the Invention
Embodiments of the present invention generally relate to methods for extracting coal bed methane with source fluid injection. Specifically, methods are provided for forming one or more laterals off a main wellbore using an approach that is economical and does not substantially damage the formation.
2. Description of the Related Art
A common method of drilling wells from the surface through underground formations employs the use of a drill bit that is rotated by means of a downhole motor (sometimes referred to as a mud motor), through rotation of a drill string from the surface, or through a combination of both surface and downhole drive means. Where a downhole motor is utilized, typically energy is transferred from the surface to the downhole motor through pumping a drilling fluid or “mud” down through a drill string and channeling the fluid through the motor in order to cause the rotor of the downhole motor to rotate and drive the rotary drill bit. The drilling fluid or mud serves the further function of entraining drill cuttings and circulating them to the surface for removal from the wellbore. In some instances the drilling fluid may also help to lubricate and cool the downhole drilling components.
When drilling for oil and gas there are many instances where the underground formations that are encountered contain hydrocarbons that are subjected to very high pressures. Traditionally, when drilling into such formations a high density drilling fluid or mud is utilized in order to provide a high hydrostatic pressure within the wellbore to counteract the high pressure of the hydrocarbons in the formation below. In such cases the high density of the column of drilling mud exerts a hydrostatic pressure upon the below ground formation that meets or exceeds the underground hydrocarbon pressure thereby preventing a potential blowout which may otherwise occur. Where the hydrostatic pressure of the drilling mud is approximately the same as the underground hydrocarbon pressure, a state of balanced drilling is achieved. However, due to the potential danger of a blowout in high pressure wells, in most instances an overbalanced situation is desired where the hydrostatic head of the drilling mud exceeds the underground hydrocarbon pressure by a predetermined safety factor. The high density mud and the high hydrostatic head that it creates also helps prevent a blowout in the event that a sudden fluid influx or “kick” is experienced when drilling through a particular aspect of an underground formation that is under very high pressure, or when first entering a high pressure zone.
Unfortunately, such prior systems that employ high density drilling muds to counterbalance the effects of high pressure underground hydrocarbon deposits have met with only limited success. In order to create a sufficient hydrostatic head in many instances the density of the drilling muds has to be relatively high (for example from 15 to 25 pounds per gallon) necessitating the use of costly density enhancing additives. Such additives not only significantly increase the cost of the drilling operations, but can also present environmental difficulties in terms of their handling and disposal. High density muds are also generally not compatible with many 4-phase surface separation systems that are designed to separate gases, liquids and solids. In typical surface separation systems, the high density solids are removed preferentially to the drilled solids and the mud must be re-weighted to ensure that the desired density is maintained before it can be pumped back into the well.
High density drilling muds also present an increased potential for plugging downhole components, particularly where the drilling operation is unintentionally suspended due to mechanical failure. Further, the expense associated with costly high density muds is often increased through their loss into the underground formation. Often the high hydrostatic pressure created by the column of drilling mud in the string results in a portion of the mud being driven into the formation requiring additional fresh mud to be continually added at the surface. Invasion of the drilling mud into the subsurface formation may also cause damage to the formation.
A further limitation of such prior systems involves the degree and level of control that may be exercised over the well. The hydrostatic pressure applied to the bottom of the wellbore is primarily a function of the density of the mud and the depth of the well. For that reason there is only a limited ability to alter the hydrostatic pressure applied to the formation when using high density drilling muds. Generally, varying the hydrostatic pressure requires an alteration of either the density of the drilling mud or the surface backpressure, both of which can be a difficult and time consuming process.
Therefore, there has been developed the technique that is called underbalanced or managed pressure drilling, which technique allows for greater production, and does not create formational damage which would impede the production process. Furthermore, it has been shown that productivity is enhanced in multilateral wells combined with the non-formation damaging affects of the underbalanced or managed pressure drilling. In this technique, a predetermined differential pressure is maintained between the pressure exerted on the formation by the column of drill fluid (plus back pressure) and a characteristic formation pressure, i.e., pore pressure or fracture pressure. There is some disagreement among those skilled in the art over the distinction between managed pressure and underbalanced drilling. Some would define managed pressure drilling as a species or sub-set of underbalanced drilling where it is often preferable to maintain the pressure exerted on the formation at some value between the fracture pressure and pore pressure of the formation. Others would define the terms in opposite fashion where underbalanced is a species or sub-set of managed pressure drilling.
The underbalanced or managed pressure technique is accomplished by introducing a lighter fluid such as nitrogen or air into the drill hole, or a combination of same or other type fluids or gases, sufficiently as manage the pressure on the formation so that fluid in the borehole does not move into the formation during drilling. One technique of underbalanced or managed pressure drilling is referred to as micro-annulus drilling where a low pressure reservoir is drilled with an aerated fluid in a closed system. In effect, a string of casing is lowered into the wellbore and utilizing a two string drilling technique, there is circulated a lighter fluid down the outer annulus, which lowers the hydrostatic pressure of the fluid inside the column, thus relieving the formation. This allows the fluid to be substantially equal to or lighter than the formation pressure which, if it weren't, would cause everything to flow into the wellbore which is detrimental. By utilizing this system, drillers are able to circulate a lighter fluid which can return up either the inner or outer annulus, which enables them to circulate with a different fluid down the drill string. In doing so, basically air and/or nitrogen are being introduced down the system which allows them to circulate two different combination fluids with two different strings.
Drilling for coal bed methane presents different conditions than drilling for oil and gas. If oil is used for drilling into the formations, the fluids may clog the permeations through the coal damaging the formation. A typical coal bed methane formation takes much longer to produce from than does an oil and gas formation. The formations must be dewatered and then the methane must separate from the coal before entering the wellbore. Uncontrolled overbalanced drilling with water would just add to the dewatering work and could possibly damage the formation. The returns from a coal bed methane formation are steady as compared to the exponential returns from an oil and gas formation. Returns from a single formation may be small relative to an oil and gas formation. Using conventional drilling and completion methods may call for ignoring smaller formations. Thus, inexpensive drilling and completion methods are advantageous. Many of the known formations are in environmentally sensitive areas making the option of drilling several conventional wells disadvantageous. Thus, for a well to be economically and environmentally viable, drilling several laterals from a single vertical or horizontal main wellbore is preferred. Coal bed methane formations are typically closer to the surface than oil and gas formations. This characteristic combined with lower reservoir pressures and a non-erosive nature compared to oil and gas wells presents the option of using drillable casing for lining all or sections of the wellbore.
Thus, there exists in the art a need for an inexpensive method for drilling a multilateral wellbore where the pressure exerted on a formation of interest by a column of drilling fluid may be controlled.